An electronic device, such as a mobile phone and a car navigation system, has had higher performance and various functions, and often includes a light-transmissible touch panel attached onto a front surface of a display element, such as a liquid crystal display. An operator presses the touch panel with a finger or a pen while looking at the display element through the touch panel, thereby switching the functions to operate the device. This touch panel is demanded to be reliably operated and not to prevent the operator from looking at the display element.
FIGS. 4 and 5 are a side sectional view and a plane sectional view of conventional touch panel 5001, respectively. Upper board 501 has a film shape and is light-transmissible. Light-transmissible upper resistor layer 503 made of resistor material, such as indium tin oxide, is provided on a lower surface of upper board 501. Lower resistor layer 504 is provided on an upper surface of light-transmissible lower board 502. Plural dot spacers made of insulating resin are arranged on an upper surface of lower resistor layer 504 by predetermined intervals. Pair of upper electrodes are provided at both ends of upper resistor layer 503, respectively. Pair of lower electrodes 505 are provided at both ends of lower resistor layer 504 and arranged in a direction orthogonal to a direction along which the pair of upper electrodes are arranged.
Spacer 505 having a substantial frame shape is provided on an outer periphery of a lower surface of upper board 501 or on an outer periphery of an upper surface of lower board 502. An upper surface and a lower surface of spacer 505 are coated with adhesion layers causing the upper and lower surfaces of spacer 505 to adhere onto an outer periphery of upper board 501 and an outer periphery of lower board 502, respectively.
Wiring board 506 having a film shape has a lower surface having wiring patterns 507 formed thereon. Wiring patterns 507 are made of copper and plated with nickel or gold. Cover sheet 508 having a film shape is adhered onto the lower surface of wiring board 506 so as to cover the surfaces of wiring patterns 507 other than the left and right end portions of the wiring patterns.
The left end of wiring board 506 is sandwiched between upper board 501 and lower board 502. Anisotropic conductive adhesive 509 allows end portions of an upper electrode and a lower electrode extending to the upper surface of lower board 502 to adhere to left end portions of wiring patterns 507, respectively, thus, electrically connecting wiring patterns 507 to both ends of upper resistor layer 503 and lower resistor layer 504 via the upper electrode and the lower electrode. Anisotropic conductive adhesive 509 includes synthetic resin and conductive particles dispersed in the synthetic resin.
Adhesive 510 is made of resin, such as acryl or rubber, and is coated onto upper board 501 and the right end of lower board 502. Adhesive 510 is allows wiring board 506 to adhere and fix to upper board 501 and lower board 502, thus providing touch panel 5001.
Touch panel 5001 is provided on a display surface of a display device, such as a liquid crystal display, thus being attached to an electronic device. Wiring board 506 is bent downward to allow the right end thereof to be connected to an electronic circuit of the electronic device, for example, with a connector or by soldering.
An operator presses the upper surface of upper board 501 with a finger or a pen while looking at the display surface of the display element through the touch panel. Then, upper board 501 accordingly sags locally to cause upper resistor layer 503 at a pressed portion to contact lower resistor layer 504.
The electronic circuit of the electronic device alternately applies a voltage to upper resistor layer 503 and lower resistor layer 504 via wiring patterns 507. The electronic circuit detects a voltage of the resistor layer to which the voltage is applied through the resistor layer to which the voltage is not applied, calculates the position of the pressed portion based on the detected voltage, and switches functions to control the device according to the position.
In conventional touch panel 5001, wiring board 506 is bent in direction D501, as shown in FIG. 4, in order to connect the right end portion of wiring pattern 507 to the electronic circuit of the device. While wiring pattern 507 is fixed with adhesive 510, a stress concentrates to portion P501 near the left end of cover sheet 508 of wiring pattern 507, and may cause portion P501 of wiring pattern 507 to have crack or breakage. This may cause unstable electrical connection between touch panel 5001 and the electronic circuit.